Tracking issue for PR #3361

[pwoolcoc]

Per @alexcrichton in this comment, I am opening a tracking issue for the plan to eventually change cargo to assume that a build.rs file in the same directory as a Cargo.toml file is a build script. Right now cargo just issues a warning when it sees a build.rs file in the same directory as a Cargo.toml file that lacks a build = ... value.

Original PR:
#3361

[pwoolcoc]

@alexcrichton hello again! now that 1.15 is out with this change, what do you think of completing the change on the current nightly? Assuming it gets in to 1.17, that would mean users would get the build = false warning for 2 stable releases before the behavior changes.

[alexcrichton]

Good question! @rust-lang/tools -- thoughts about whether we're far enough along to implement this change?

[nrc]

Could you clarify what exactly the change is? My understanding is that currently if build = false and there exists a build.rs in the project root, then we warn and we would change this to a hard error? Or does the warning happen when there is no build entry and a build.rs file and we would change this to silently accepting?

Is this a backwards incompatible change?

[alexcrichton]

@nrc sure yeah, the state of play is this:

• We'd like to automatically infer build.rs as the build script
• This is backwards-incompatible to projects which have build.rs but it's not a build script
• We added the ability to specify build = false to Cargo.toml to work around this.
• Cargo currently warns if build.rs exists, isn't a build script, and build = false is missing.
• Older Cargos will reject build = false as an invalid manifest.

The concrete change to make here is to automatically infer build.rs as a build script unless otherwise configured. This would also entail removing the warning. Crates which have build.rs, it's not a build script, and haven't configured build = false will likely break.

A preliminary list of crates with build.rs but not having it as a build script were compiled. This may not be an exhaustive list.

[nrc]

How does Cargo know if build.rs is a build script or not?

To be clear, this is only crates with build.rs in the root? Or would this break for src/foo/build.rs too?

[alexcrichton]

Cargo does no inference based on file contents, just location. And yeah it's only build.rs at the root (next to Cargo.toml), which is quite rare in practice.

Cargo's proposed logic is that if build.rs exists next to Cargo.toml it's automatically inferred as a build script, no configuration necessary.

[nrc]

Thanks for the clarifications!

I think this is OK to go ahead with - we'll have had 2 whole warning cycles and the fix is pretty trivial.